1. Field of the Invention
The invention relates to a superconducting fiber of a superconducting fiber bundle, in which the outer surface of a carrier fiber is surrounded by a superconducting layer.
2. Description of the Prior Art
In the further development of energy technology, with a view to nuclear fusion and superconducting generators, transport technology (magnetic levitation train), environmental technology (coal desulfurization) and high-energy physics, high-field magnets are required which, due to economic aspects, can be produced only on the basis of superconductors.
Particularly for such applications, suitable carrier fibers (for example C, B or steel) having a high modulus of elasticity were coated with a superconducting material and combined into fiber bundles which contain any desired number of individual fibers. The carrier fibers serve as a matrix strong in tension and as a substrate for the application of the superconducting layer. Promising superconducting materials, for example niobium carbonitrides and niobium oxycaronitrides of the general formula NbC.sub.x N.sub.Y O.sub.z (x+y+z smaller than or equal to 1) have been disclosed. Niobium compounds of this type are distinguished by high critical temperatures, high critical magnetic fields and high critical current densities. They can be applied to the carrier fiber by a chemical vapor deposition process (VD) in which the niobium is deposited as a thin film by reaction of NbCl.sub.5 with H.sub.2 in the presence of carbon-containing and nitrogen-containing gases. The CVD process is here carried out either in a single stage (simultaneous deposition of Nb and carbonitriding) or in two stages (deposition of Nb and carbonitriding being carried out successively).
German Auslegeschrift No. 2,856,885 and the publication "Chemical Vapor Deposition of Superconducting Niobium carbonitride Films on Carbon Fibers", K. Brennfleck, M. Dietrich, E. Fitzer, D. Kehr, Proc. of CVD--7th International Conference, 300-314, The Electrochemical Society, Princeton, N. Y. (1979), have disclosed such a two-stage CVD process. Moreover, further improved processes which are suitable for the production of superconducting fiber bundles have been proposed in related application Ser. No. 518,381, and in the publication "CVD Processing of Carbon Fiber Superconductors", M. Dietrich, C. H. Dustmann, F. Schmaderer, G. Wahl, presented at the Applied Superconductivity Conference, 1982, Knoxville, Paper MB 2. Superconducting fiber bundles produced by these improved processes have a homogeneous coating of the individual fibers of the fiber bundle. Their superconducting layers are extremely fine-grained, so that high critical magnetic fields and high critical current densities were obtained.
In the production of superconducting fibers, however, it has been found that it is very difficult to achieve adequate adhesion of the superconducting layer to the carrier fiber. This problem arises more or less with every type of carrier fiber or type of superconductor. One reason for this is the different coefficients of thermal expansion of the carrier fiber and of the superconducting layer. Another cause is to be seen in the fact that, during the coating of the carrier fibers by the CVD process, reactive gases, such as hydrogen gas or ammonia, attack the fiber surface or form undesired hydrocarbon compounds on the fiber surface and make good adhesion of the superconducting layer more difficult.
The inadequate adhesion of the superconducting layer to the carrier fiber results in cracks in the superconducting layer and, under certain circumstances, causes it to peel off. This is a disadvantage, since interruptions in the superconducting layer by cracks or gaps entail a reduction of the critical current density of the superconducting fiber.
Application of the above-mentioned two-stage CVD process was able only partially to solve these problems in the case of carrier fibers consisting of carbon fibers of high tensile strength (HT carbon fibers). By contrast, satisfactory coating of high-modulus fibers (HM carbon fibers), that is to say carbon fibers with particularly high graphitization and consequently a high modulus of elasticity, has hitherto not been possible. HM carbon fibers are, however, distinguished by better mechanical compatibility with niobium carbonitride or niobium oxycarbonitride, as compared with HT carbon fibers, so that their use as the carrier fiber is particularly desirable.